1. Field of the Invention
The present invention relates to a method for rounding and flattening hollow cylindrical parts which are out of round or are not flat due to non-uniform internal stresses.
2. Description of the Related Art
It is well known that steel parts can distort after heat treatment due to internal stresses created in the part during the heat-treat process. For instance, when a carbon steel part is quenched from above the austenitizing temperature, martensite is formed. The transformation of austenite to martensite is accompanied by an expansion in volume. As a result of the volume expansion, internal stresses are induced into the part. Any irregularities in the internal stresses can cause part distortion. In hollow cylindrical steel parts, the distortion can cause the parts to go out-of-round or cause the parts to lose flatness similar to a potato chip. The distortion is either tolerated in the application for the part or commonly the distortion is honed or ground out of the part at great expense. Thus, there is a need for a more cost effective method for rounding and flattening hollow cylindrical parts which are out of round and/or are not flat.
Methods have been proposed for straightening truck structural members. Methods have also been proposed for straightening out of true shafts. Prior methods have been used where bent heat-treated shafts are straightened by back-bending. Methods for truing out bent shafts have been used wherein forces are applied to a bent shaft in a locally limited area, whereby these forces are sufficient to locally strengthen the shaft to cause compressive residual stress in a surface layer zone of the shaft for reducing the out of true bending of the shaft.
The compressive residual stress may be generated in the surface layer zone of the bent shaft in various manners. Examples of means that have been known for achieving compressive residual stress in a surface layer zone include case hardening, induction hardening, laser beam hardening, nitridation, and deep rolling. The compressive residual stresses are induced only in a surface layer zone of the shaft such that the induced compressive residual stresses cause a corresponding deformation of the shaft. The direction of this deformation depends on which specific surface areas of the shaft have induced compressive residual stresses. It is reported that in order to achieve a desired truing effect, the compressive residual stresses should be induced in the shaft in a defined locally bounded area. This may be achieved by means of a locally limited hardening process, or by means of a locally limited deep rolling operation.
Case hardening in known methods required selective masking of the shaft to prevent surface portions of the shaft which must remain non-hardened from becoming hardened in the case hardening process. The case hardening method of hardening is both energy and labor intensive, and is therefore quite expensive. Nitridation suffers from similar drawbacks. In induction hardening, the shaft to be hardened is placed inside a coil through which a rapidly alternating current is flowing. In this method, it may also be difficult to prevent surface portions of the shaft which must remain non-hardened from becoming hardened in the induction hardening process. As such, induction hardening is also expensive and time-consuming. Deep rolling operations require complicated equipment and therefore, are also quite expensive.
Therefore, while methods have been proposed for straightening out of true shafts, there exists a need for more cost effective methods for rounding and flattening hollow cylindrical parts which are out of round and/or are not flat.